Optically variable devices (OVD) are a common protective element on various types of documents (e.g. identity cards, passports, visas, bank cards)—see the book “Optical Document Security”, ed. by R. L. Van Renesse, Artech House, (1998). Holograms and other diffractive elements are mainly used, because their protective value is based on complexity of micron and submicron structures. Manufacturing is a complicated and expensive process whose final result is a master hologram—a single, unique prototype. To make protection commercially acceptable, the master hologram is copied and multiplied, resulting in a replica shim used for embossing into a plastic foil, which is then integrated into a document using a hot tool. The final result is a series of documents possessing exactly the same protective OVD. This is a significant drawback, because, if the OVD is counterfeited, a large number of fake documents can be manufactured.
As a result, there is ongoing research for a simple and affordable document individualization method. This makes counterfeit much harder, because each and every document has to be copied individually, i.e. large scale production of false documents becomes impossible. However, the trivial individualization by simply printing numbers will not work, because it is too simple and affordable, if using modern printing technologies (e.g. laser printing). Therefore the individualization-bearing features must possess a significant amount of complexity together with strong, unrepeatable, individual properties. They have to be comparable in its uniqueness with biometric characteristics, such as: fingerprints, iris and retina pattern, but significantly more complex and miniscule. Currently used OVD security methods are not well suited for individualization (fingerprinting), as this will significantly increase the production prices.
Attempts to obtain “fingerprint” documents are based on the idea of physical one-way functions (C. Boehm, M. Hofer, “Physically unclonable functions in theory and practice”, Springer, 2013)—which are physical devices simple to manufacture, yet extremely difficult to reverse engineer and copy. Random structures can be highly significant for document security, because they offer simple and cheap production, almost impossible re-origination and unique features. It was proposed to tag documents with randomly dispersed objects such as metal, fluorescent or optical fibers (van Renesse book, and references therein).
Natural fibrous structure of paper-based substrates was used (J. D. R. Buchanan, R. P. Cowburn, A-V. Jausovec, D. Petit, P. Seem, G. Xiong, D. Atkinson, K. Fenton, D. A. Allwood, M. T. Bryan, “‘Fingerprinting’ documents and packaging”, Nature 436, (2005) 475). Laser beam was scattered from the paper surface and its statistics was observed and recorded. This however requires a large scale scanning of the document surface which is a slow process, and paper structure may be strongly affected by printing and everyday usage.
Yet another technique was described in R. Pappu, B. Recht, J. Taylor, N. Gershenfeld, “Physical One-Way Functions,” Science 297, (2002) 2026-2030, where mesoscopic scattering from disordered array of plastic spheres embedded in a transparent substrate was used to construct physical one-way function. The response of the system strongly depends on the illumination direction, again producing unique individual characteristics. The proposed method is limited by the physical requirements for the mesoscopic scattering, resulting in a 10 mm×10 mm sized tag, with 2.5 mm thickness, which is unsuitable for the modern plastic card technology. Furthermore, the dimension of scattering particles is rather large—500-800 μm in diameter, with 100 μm average spacing—resulting in a bulky system which can be reverse engineered by techniques like micro-tomography.
It is a common knowledge that certain natural characteristic of living creatures are essentially complex and hard to reproduce. This was first realized by Benjamin Franklin who used this for document protection (Farley Grubb, “Benjamin Franklin and the birth of the paper money economy”, Essay based on Mar. 30, 2006 lecture, published by Federal Reserve Bank of Philadelphia). He made casts of plant leaves (correctly recognizing the uniqueness of their venation) and used them to print the first dollar bills. Due to further technological advancements, Franklin's method became obsolete, and was replaced with different printing techniques, such as: intaglio, guilloche, watermark, holograms, etc.
Complexity of natural structures was observed in art, too. Japanese painters used fish printing (gyotaku) to directly transfer fish features, instead of painting them. Later, Leonardo Da Vinci directly printed leaf venation on the paper, while Dutch painter Otto Marseus Van Schrieck transferred butterfly wing scales to his canvases (S. Berthier, J. Boulenguez, M. Menu, B. Mottin, “Butterfly inclusions in Van Schrieck masterpieces. Techniques and optical properties”, Appl. Phys. A, 51-57, (2008)). Today, all the techniques have the common name: nature printing (R. Newcomb, “Method for producing nature prints”, U.S. Pat. No. 4,279,200 A, (1981), C. F. Cowan, “Butterfly wing-prints”, J. Soc. Biblphy. Nat. Hist., 4 (1968) 368-369, D. G. Edwards, “A receipt for taking figures of butterflies on thin gummed paper”, in Essays upon natural history and other miscellaneous subjects, pg. 117).
Patents WO 2007031077 (A1) March 2007, C. Hamm-Dubischar, “Inorganic marking particles for characterizing products for proof of authenticity method for production and us thereof” and DE10238506 A1, 3/2004, H. Rauhe, “Producing information-bearing micro-particulate mixtures involves defining code that can be implemented using natural or subsequently applied particle characteristics selected from e.g. morphology”, disclosed an idea for document protection which uses natural complexity of aquatic organism inorganic shells (like diatoms and radiolarians) according to characteristics of their surfaces. The practicing method is, however, not disclosed. Another problem is that the optical effects are not very pronounced, and the complexity can be observed only at the sub-wavelength levels, using electron microscopy. Technique for estimating the degree of complexity was not described, either. Variation among the specimens of the same species is rather small. In that respect, the method can be used only for the forensic level of document authentication.
Recently, there was a significant amount of research aimed at using the principles of optics in nature for document protection—biomimetics (J. Sun, B. Bhushanand J. Tong, “Structural coloration in nature”, RSC Adv., 2013, 3, 14862-14889, B. Yoon, J. Lee, I. S. Park, S. Jeon, J. Lee, J-M. Kim, “Recent functional material based approaches to prevent and detect counterfeiting”, J. Mater. Chem. C 1, (2013) 2388-2403). Variability of biological structures was also observed (L. P. Biro and J-P. Vigneron, “Photonic nanoarchitectures in butterflies and beetles: valuable sources for bioinspiration”, Laser Photonics Rev. 5, No. 1, 27-51 (2011)). Biotemplating was used to manufacture butterfly scale-like structures using metals (S. Sotiropoulou, Y. Sierra-Sastre, S. S. Mark, and C. A. Batt, “Biotemplated Nanostructured Materials”, Chem. Mater. 2008, 20, 821-834).
The randomized systems described above must be machine-inspected, based on radiation scattering with consequent optical or microwave detection (in the case of metal inclusions). Recorded pattern is encrypted and stored in a central repository or on the document, itself. Public key encryption method is used, as described in the report: “Counterfeit deterrent features for the next generation currency design”, Committee on Next-Generation Currency Design, National Materials Advisory Board, Commission on Engineering and Technical Systems, National Research Council, Publication NMAB-472, (1993), Section: Random Pattern/Encryption Counterfeit-Deterrence Concept, pg. 74-75, and Appendix E: “Methods for authentication of unique random”, pg. 117-119. A technique is based on two keys: a secret one, used for encryption, and a public one, used for the decryption.
All the methods used a complexity of natural structures but their variability remained completely unused in the context of the document protection. Document variability was rather attained by randomly dispersing particle- or thread-like entities across the document, as described in the patent literature (U.S. Pat. No. 8,408,470B2, 2013, N. Komatsu, S-I. Nanjo, “Object for authentication verification, authentication verifying chip reading device and authentication judging method”).